refactor(adapter): split trap/page_table/smp/entry by concern; trim comments

crates/adapter-riscv/src/mmu/mod.rs

@@ -7,10 +7,8 @@ pub use page_table::{
     create_user_root, init_kernel_page_table, map_user_page, translate_user, unmap_user_page,
 };
 
-/// The kernel lives in the upper half of the address space: all of RAM and
-/// the MMIO windows appear at their physical address plus this offset (the
-/// start of Sv39's upper half), and the kernel image is linked there too,
-/// so one offset converts any kernel address. Must match linker.ld.
+/// Direct-map offset: RAM, MMIO and the kernel image sit at physical + this
+/// (start of Sv39's upper half). Must match linker.ld.
 pub const PHYS_OFFSET: usize = 0xFFFF_FFC0_0000_0000;
 
 /// Where the kernel sees physical address `pa`.
@@ -18,8 +16,7 @@ pub const fn phys_to_virt(pa: usize) -> usize {
     pa + PHYS_OFFSET
 }
 
-/// The physical address behind kernel address `va` (a static, the boot
-/// heap, a page table). Only valid for addresses in the direct map.
+/// The physical address behind a direct-map kernel address `va`.
 pub const fn virt_to_phys(va: usize) -> usize {
     va - PHYS_OFFSET
 }